State of the art nonvolatile memory devices are typically constructed by fabricating a field effect transistor (FET) in a silicon substrate. The field effect transistor is capable of storing electrical charge in the gate electrode of the transistor. The gate electrode is known as a floating-gate electrode and is separated from the silicon substrate by a dielectric layer. Data is stored in a nonvolatile memory device by changing the threshold voltage of the field effect transistor. The threshold voltage of the FET is shifted to a higher or lower value through the storage of electrical charge in the floating-gate electrode of the field effect transistor. For example, in an N-channel transistor, an accumulation of electrons in a floating-gate electrode creates a high threshold voltage. Conversely, the removal of electrons from the floating-gate electrode creates a low threshold voltage.
The nonvolatile memory device is "read" by applying a voltage to the drain terminal and the control gate electrode, while keeping the substrate at ground potential. A sense amplifier is connected to the source terminal to measure any current flowing through the device. The read voltages are chosen such that no current flows through the device if it is programmed to the high voltage level. However, a measurable current flows if the device has a low threshold voltage level.
The logic state of the nonvolatile memory device is determined by presence or absence of a measurable current when read voltages are applied. Conventionally, the detection of current flow through the FET is defined as a logic "0" state. Conversely, the absence of current flow in the FET is defined as a logic "1" state.
In the operation of a nonvolatile memory device, it is important that the charge placed on the floating-gate electrode remain at a constant value until the memory device is deliberately reprogrammed. The ability of a nonvolatile memory device to maintain charge on the floating-gate electrode is characterized as the data retention capability of the memory device. Ideally, a nonvolatile memory device should have infinite data retention capability. However, many nonvolatile devices exhibit less than optimal data retention because of unwanted charge dissipation from the floating-gate electrode. Typically, negative charge on the floating-gate electrode can be dissipated through diffusion of ionic contaminates through the overlying passivation layers and into the floating-gate electrode. For example, sodium ions impinging on the floating-gate electrode result in data loss by shifting the logic state of the nonvolatile memory device from a logical "1" to a logical "0".
Poor data retention can also arise in a nonvolatile memory device by the generation of a nonuniform electric field near the floating-gate electrode. A nonuniform electric field can result from sharp corners and protrusions of the floating-gate electrode. The concentration of electric field lines at the corners of the floating-gate electrode is exacerbated when the floating-gate electrode is fabricated from a material having a metallic content, such as a silicide material or a pure metal. Nonvolatile fabrication process which use refracting metal silicides to increase the electrical conductivity of the device can have severely nonuniform electrical fields around the floating-gate electrode. High field concentration can disrupt the electrical charge on the floating-gate electrode and shift the threshold voltage of the FET. Accordingly, further development of nonvolatile memory device and design fabrication processes are necessary to provide nonvolatile memory devices having improved data retention.